14.1 Dust Storms: Simplified Algorithm

HYSPLIT contains two different wind-blown dust emission algorithms. The original procedure was developed for applications in Asia, the Middle East, and Saharan dust, while the newest procedure was developed for use over North America. For both versions, the dust emission algorithm is invoked by setting the namelist parameter ICHEM=3 in the file SETUP.CFG and adding two additional fields in the CONTROL file on the emission location line. See the NOAA web page for more information.

The original dust emission algorithm (an option not currently available) was based upon describing the vertical dust flux as a fraction of the horizontal transport:

Q = K ρ g-1 U* (U*2-U*t2) A

where the soil emission factor K and threshold friction velocity U*t varied by soil type and land use. This approach proved to be impractical in locations where these details are unknown, hence a simplified method was applied:

Q = 0.01 U*4 A.

Dust emissions occur at locations where a "hardwired" threshold friction velocity (28 cm/s) is exceeded. The GUI option Special Runs / Dust-storm invokes a pre-processor to read the CONTROL file with the three starting points (bottom left, upper right, resolution) to create the final CONTROL file for dust emissions by finding all the desert area land-use locations within that domain. The potential dust emission points are defined in the CONTROL file by the two additional fields after the release height: the threshold friction velocity and the emission area (m2) centered about the emission point. To indicate that the simplified emission algorithm is being used, the threshold friction velocity field is set to -1.0.

The above procedure cannot be used in the continental U.S. with the default landuse files because there are no desert locations covering a full one-degree grid point. However, optional half-degree files are provided with the HYSPLIT distribution in the /hysplit4/bdyfiles/bdyfiles0p5 directory. To use these files instead of the default ones, you must copy the ASCDATA.CFG file from the bdyfiles0p5 directory to the /hysplit4/working directory. The contents of this file are read during the HYSPLIT simulation initialization step and define the contents and structure of the three data files within that directory: LANDUSE.ASC, ROUGHLEN.ASC, and TERRAIN.ASC. If your HYSPLIT installation is different than the default, another option would be to edit your ASCDATA.CFG file to point to the correct directory, with a data resolution of 0.5, and the number of latitude and longitude data points: 360 and 720.

To demonstrate the dust-storm computation, we will focus on an event that occurred in Utah from 30-31 March 2010. Open the Setup Run menu and retrieve the pre-configured control file CONTROL_dust0.txt from the \Tutorial\dust directory. If this example is being run through the web rather from a CD, it will be necessary to download the meteorological data file NAM20100330.bin.

Note that in the Concentration Grid menu the sampling start time is set for 00 00 00 06 00, which means that sampling will start 6 hours after the start of the simulation, for a 24-hour duration sample. The model results will be compared with the daily averaged (local midnight to midnight) AirNow measurements. Then save to exit.

Next open the Advanced / Concentration / menu #10 and check the Enable PM10 dust storm emission algorithm radio-button. Save to exit and then open menu #4 to increase the particle emission rate to 100000 with a maximum of 50000. These changes can also be invoked by retrieving the SETUP_dust.txt file. Save the changes to exit.

Now go ahead and press the Special Runs / Dust-storm menu which configures the control file and runs the simulation. Using the half-degree resolution data file, 26 dust emitting locations are identified. When the run completes, open the Concentration / Displaymenu and set the units conversion to 1.0E+06 to convert grams/m3 to ug/m3. Also set the contour interval to fixed values of 500 200 100 50 20 10, then execute the display to show the dust plume.

The calculation can be compared to the AirNow measurements in the Salt Lake City area for that day. Open the Utilities / Convert to DATEMmenu and set the measured data input file to AirNow_dust.txt and with the conversion factor of 1.0E+06. The statistical results show under-prediction by about a factor of 10 and a correlation coefficient of 0.48. which is confirmed by viewing the scatter diagram.

The dust storm modeling approach shown here required no additional information besides the landuse files supplied with HYSPLIT. In the next section, a customized CONTROL file will be used that is predefined with all potential dust emission locations.